FIG. 13 is a schematic front elevational view of a fuel injection valve 1, in its entirety, for use on an automobile. The fuel injection valve 1 has a nozzle holder 2 and a fuel injection nozzle 3 of metal which is held on a tip end of the nozzle holder 2. The reference numeral 4 represents an inlet port for drawing in a fuel.
FIG. 14 is a schematic fragmentary vertical cross-sectional view of a tip end of the fuel injection nozzle 3. As shown in FIG. 14, the tip end of the fuel injection nozzle 3 is constructed as a hollow portion in which a nozzle needle 5 is inserted. When the nozzle needle 5 is displaced, the hollow portion of the fuel injection nozzle 3 is brought into or out of fluid communication with a plurality of injection holes 6 (only two are shown in FIG. 14) defined through the tip end of the fuel injection nozzle 3, opening or closing a fuel passage.
Recently, the injection holes 6 are formed as through holes, using a laser beam. In other words, a laser machining process is performed to form the injection holes 6.
The laser beam is applied to the tip end from outside the tip end. The laser beam melts the wall of the tip end from an outer wall surface thereof toward an inner wall surface thereof. Finally, the inner wall surface is melted to form an injection hole 6. A laser machining apparatus which applies a laser beam for such a machining process is disclosed in Japanese Patent No. 2623296, for example.
When the above laser machining process is performed, the laser beam which is focused may not be of a truly circular cross-sectional shape, tending to result in a reduction in the accuracy of the diameter and roundness of the injection holes. For the purpose of eliminating the above drawback and increasing the machining accuracy, Japanese Laid-Open Patent Publication No. 2001-150248 proposes an electric discharge process for finishing a small hole formed by a laser machining process, and Japanese Laid-Open Patent Publication No. 2001-212685 proposes a process of machining a workpiece by applying an ultrashort-pulse laser beam such as a femtosecond laser beam or the like after the workpiece has been preheated by a first laser beam.
The prior art disclosed in Japanese Laid-Open Patent Publication No. 2001-150248 will briefly be described below with reference to FIGS. 15 and 16.
First, as shown in FIG. 15, a laser machining head 101 is moved to a position directly above a hole machining position on a workpiece 102. The laser machining head 101 applies a laser beam 103 to form a pilot hole 104 through the workpiece 102.
Then, as shown in FIG. 16, an electric discharge machining head 105 is guided along a moving base 107 to a position directly above the pilot hole 104. Then, an electrode 106 is lowered and performs an electric discharge machining process to enlarge the pilot hole 104 into a through hole having a predetermined diameter.